Background: Ischemic injury following myocardial infarction activates a complex healing process involving cardiac fibrosis, which is characterized by an excessive deposition of extracellular matrix (ECM) due to an imbalance of collagen synthesis and turnover, potentially leading to cardiac dysfunction and heart failure. Therefore, we aimed at exploring putative mediators involved in fine-tuning collagen turnover and ECM dynamics in order to enable the development of novel anti-fibrotic therapeutic strategies.

Methods: We utilized single-cell RNA sequencing data derived from the non-cardiomyocyte fraction of murine hearts on day 0, 1, 3, 7, 14, and 28 following myocardial infarction. Molecular signatures of healthy and diseased fibroblasts were obtained by gene ontology analysis of differentially expressed genes. 

Results: Besides a time-dependent switch in cardiac fibroblast plasticity following myocardial infarction, analysis of differentially expressed genes revealed a continuous upregulation of Mannose receptor C type 2 (MRC2) in cardiac fibroblasts. Consistently, in silico analysis of publicly available bulk RNA sequencing data of isolated murine cardiac fibroblasts upon ischemic cardiac injury as well as TGFbeta stimulation of human cardiac fibroblasts in vitro (p < 0.05) confirmed increased MRC2 expression within a profibrotic environment. While MRC2 has been primarily studied for its role as an endocytic collagen receptor in lung and renal disease, we investigated its function in pathological cardiac fibrosis by siRNA-mediated silencing of MRC2 in human cardiac fibroblasts. Using a flow cytometry–based fluorescence-labeled collagen uptake assay, we demonstrated that cardiac fibroblasts with diminished levels of MRC2 upon siRNA-mediated knockdown internalized less collagen (all p < 0.01). In turn, increased MRC2 level upon TGFbeta stimulation resulted in enhanced collagen turnover (all p < 0.01), implicating that a disease-dependent MRC2 upregulation in cardiac fibroblasts upon ischemic injury might represent a compensatory mechanism for extracellular matrix renewal. In addition to its cell-intrinsic capacity to internalize collagens, we assessed whether MRC2 might also affect cardiac fibroblast migratory and contractile capacity by an interaction with ECM components. Indeed, cardiac fibroblasts upon MRC2 silencing exhibited reduced cell migration (p < 0.01) as well as an impaired ability to promote ECM contraction (p < 0.05) compared to MRC2 expressing cardiac fibroblasts, mainly due to attenuated binding to specific collagen subtypes amongst a bundle of different ECM components. Surprisingly, silencing of the cell surface receptor MRC2 in cardiac fibroblasts not only impaired the cell-intrinsic interplay with ECM components, but also reduced protein abundance of lysosomal enzymes and matrix metalloproteinases involved in collagen degradation, potentially by intracellular signaling. 

Conclusion: Taken together, our findings suggest that high levels of the cell surface receptor MRC2 orchestrate ECM-turnover and renewal accompanied with improved mechanotransductional abilities thereby promoting cardiac fibroblasts mobility within the fibrotic tissue, which should be further investigated as a putative anti-fibrotic approach.